Cancer cells often express high levels of several heat shock proteins (HSPs) which augment the aggressiveness of these tumors and also allow the cells to survive lethal conditions, including killing by therapies. In addition to conferring resistance to treatment, elevated HSP expression also facilitates cancer by inhibiting programmed cell death and by promoting autonomous growth.
Among the major HSPs are Hsp90 and Hsp70, proteins that act in an interconnected but also distinct fashion to regulate the malignant phenotype. Hsp90 maintains the transforming capacity of several onco-proteins, among which HER2, AKT, RAF1, IGF-IR, and HIF-1, function facilitated by Hsc70, the constitutive Hsp70, the inducible Hsp70 isoforms. Upon Hsp90 inhibition, the Hsp90-client onco-proteins become destabilized and are degraded by a proteasomal pathway (FIG. 1a). The transcription factor HSF-1, the master regulator of heat shock response, is another Hsp90 client, and unlike onco-proteins, it becomes activated when Hsp90 is inhibited. HSF-1 activation leads to an increase in Hsp70 levels, a feed-back response that limits the potency of Hsp90 inhibitors in certain tumors. Hsp70 in itself is a powerful anti-apoptotic molecule, suggesting that inhibition of both intrinsic and extrinsic apoptotic pathways by increased Hsp70 levels may be responsible for reducing the effect of Hsp90 inhibition. In addition to inhibiting apoptosis and assisting Hsp90, Hsp70 and its highly homologous cytosolic isoforms, serve many other overlapping chaperone functions and in some cases can substitute for each other.
The Hsp90 multi-chaperone complex, also called the Hsp90 super-chaperone machinery, has important roles in the development and progression of pathogenic cellular transformation through regulation of several malignancy driving and supporting client proteins. The activity of the Hsp90 multi-chaperone system is maintained and executed by a complex system of chaperones. The Hsp70s (constitutively expressed Hsc70 and the heat inducible Hsp70-1 and Hsp70-6) participate in the preliminary steps, whereas Hsp90 participates in the later stages (FIG. 1a). Their function requires a multitude of co-chaperones, such as the Hsp70-regulators, Hsp40, Hsp110, BAG and HIP; HSP-organizing protein (HOP), involved in the formation of the intermediate molecular chaperone complex where the client is passed from Hsp70 to Hsp90, and others such as p23, cdc37 and immunophilins, acting at the final or mature Hsp90 complex (FIG. 1a). Inhibition of the Hsp90 machinery through agents that act by direct binding to the regulatory ATPase pocket of Hsp90, such as geldanamycin (GM) and the PU derivatives PU24FCl and PU-H71 (FIG. 1b), interferes with the formation of mature complexes, directing the client proteins towards proteasomal degradation (FIG. 1a). Interestingly, reduction in the activity of Hsp90, but not in the expression of Hsp70 or the co-chaperones HOP, HIP, p23, and Hsp40, was reported to dramatically activate HSF-1. An intriguing outcome of this observation is that HSF-1 activation does not require Hsp70, unlike the onco-protein clients of the Hsp90 machinery.
Whereas the significance of direct Hsp90 inhibition is now well understood, and has been harnessed in the development of small molecule inhibitors currently in clinical evaluation for multiple cancers, little is known about alternate ways to intervene in the activity of the Hsp90 machinery. Interfering with the chaperone machinery in ways other than direct Hsp90 inhibition may differentiate between several of its functions, and confer specific biological activities.